In the nonwoven and paper industries, webs of material are produced at very high rates of speed. An important consideration in such manufacture is the monitoring of the formation of the web. The quality of the non-woven paper material is always important to the quality of the end product produced.
Formation to most web manufacturers is the degree of uniformity achieved in the distribution of the fiber mass within the web. A well formed web is one where the fibers are randomly dispersed throughout. Well formed webs are usually stronger and have better appearance, which is important to the end user.
Good formation is especially important to non-woven and paper manufacturers. Poor formation not only adversely effects a web or sheet appearance, but also effects its printability and other qualities.
The traditional methods of classifying the formation of non-woven webs and paper sheets has been with human inspection and off-line formation sensors. The describing of a given web or sheet as being good, not bad, flocky, streaky, etc., is very relative and subjective. Studies in general have shown very poor agreement between visual and off-line formation sensors.
The problems with off-line formation sensors is the time between sample and results and also obtaining good profile measurements. At the speeds most modern non-woven and paper manufacturers run thousands of yards of material can be off quality before the results from the lab are obtained.
Studies made of variations in formations of paper webs have shown more variations across the web versus downweb. Therefore, the need for continuous updating of formation profile measurements is important.
To help eliminate the problems with human inspection of webs and the use of off-line sensors, manufacturers are starting to use single point formation sensors on-line. Some are fixed on the machine while others are slowly traversed across the web. Most of these sensors use the principle of comparing the light intensity of a small area, 1 mm or less, with a larger area, say of 30 mm diameter or larger. A formation index is based upon the intensity variation in the small area as compared to the average light intensity of the larger area.
The problem to date with these on-line single point formation sensors has been with the repeatability of the results with changing machine speeds and products. Another problem has been in getting rapid formation profile measurements across the web in less time than, for example, a minute of operation.
A further improvement to these on-line single point formation sensors is the use of a linear array CCD (charged coupled device) camera which looks at the variations in light intensities across the entire moving web without traversing. This technology was an outgrowth of using linear array CCD cameras to inspect for defects in moving webs. Defects like holes or dark spots would cause the light intensity at a given area in the web to exceed from its normal range and cross a given light or dark threshold, thus signaling a defect has been detected. One such system is being marketed by Albany International, assignee of the present application, under the name WEBSPEC.TM..
In such a system an automatic, high-speed visual web inspection system is provided. A full web width high-intensity light source is used to shine through the moving web. A linear CCD camera with a zoom lens is focused on the streak of light passing through the web. The camera contains a given number of solid-state, photo-sensitive diodes used to detect the variation in light intensities across the web. An analog electrical signal, proportional to the light intensity detected by the respective photo-sensitive diodes, is sent to the computer system. Normally, all the photo-sensitive devices in the camera are used to scan the web once. Therefore, if the camera contains 2,048 devices, the number of picture elements or "pixels" per scan would equal 2,048.
The analog signals from the devices are converted to digital for binary and digital image analysis by the computer system. The analog signals from the camera are normalized, compared to threshold settings and processed with a 68000 microprocessor. An information summary on the type, size and location of the defects detected can be either stored on disk or a hard copy made using a printer and a flagger is activated to place a flag adjacent a defect in the downweb direction.
While such a defect identifier has proven eminently satisfactory for defect detection, another aspect of web analysis involves, as aforenoted, that of the formation of the web. The formation quality of paper is defined as the degree of uniformity achieved in the distribution of fiber mass. A well formed sheet is one where the fibers are very randomly dispersed throughout the entire sheet. Good formation is essential during papermaking because it strongly affects the appearance and printability of the sheet. In general, poor formation adversely effects other sheet qualities. Well formed paper is stronger, more uniform, easier to dry and allows the paper to run better.
Describing a given paper sample or sheet as good, not bad, poor, floccy, streaky, etc., indicates that formation is very subjective. Studies by the Institute of Paper Chemistry (Paper Trade Journal, May 30, 1984) concluded that reproducibility of visual formation gradings between different observers was relatively poor. A better formation grading was obtained between a Thwing-Albert formation tester and the average of the visual results.
Because of these difficulties and the importance of good formation to the papermaker, an effort has been made to develop a so-called "standard formation tester or sensor" which could be used either on or off line. Some of the testers developed over the years include the QNSM, NUI, MKS, Thwing-Albert, Microscanner.TM., Toyoseiki, Lippke and more recently, the Opti Pak.TM.. All have been used by papermakers to help quantify the formation they were obtaining and to make quality improvements.
However, there was a need for a full width which can run from 12-35 feet wide, real time formation sensor for paper machine applications which has not heretofore been achieved. This is especially true, since on most paper machines, variation in formation across the web (C.D. or X.D.) is much greater than in the downweb direction.
To meet this need there was developed a device for web formation analysis which is set forth in U.S. patent application Ser. No. 159,830, filed Feb. 24, 1988, entitled "Method And Apparatus For Analyzing A Web Of Material", which is commonly assigned to Albany International Corporation, the assignee of the present application, the disclosure of which is incorporated herein by reference.
The system which is entitled the FORMSPEC.TM. system by Albany International involves the use of a linear array CCD camera to measure the variation of transmitted light through a moving web. The light intensities across the entire web are converted into analog electrical signals and, in turn, digital signals. The linear array CCD camera is effective in determining differences in the formation of the paper web. A formation index is generated from the information generated by electronic scanning. The system measures the light intensity across the web of material which can run from 12-18 feet in width.
In such web analysis, it is accordingly important that the light source not only be of high intensity, but also uniform. Variation of the light intensity can be erroneously construed as variations in the web formation leading to improper results. While the use of custom-made lighting is possible, such as twelve foot lengths of high intensity aperture bulbs, such lighting is expensive and suffers from a fall off in intensity at its ends. It is therefor desirable that a light assembly be provided which is effective in operation but relatively inexpensive in cost of manufacture.